1. Field of the Invention
The present invention relates to remote data access via a network. In particular, the invention relates to caching network data and a system for realizing the control thereof.
2. Description of the Related Art
In recent years, companies that offer services of Application Service Provider (ASP) type have appeared. The advantages of ASP are that client users' systems can be downscaled because the provider's data center holds all data in its databases or the like and the clients can use latest applications at low cost. The service-offering companies can integrate a plurality of clients' systems and reduce the investment in their ASP facilities. Further advantage is that the clients can reduce the data management cost because the data center performs data backup and related operations. From these advantages, a broad range of services of ASP type has lately been available, and so does in the sphere of medical information.
Technique for enabling such services for data stored on disk media, which is called ISCSI, has lately attracted attention. The iSCSI is a technique of passing SCSI commands which are disk input/output commands over a TCP/IP network. The iSCSI specifications are under discussion by a working group of the Internet Engineering Task Force (IETF) which is a community for establishing standards and specifications on the Internet and the draft-level ISCSI specifications have so far been published. One of the available storage products supporting the iSCSI protocol is IBM Total Storage IP Storage 200i supplied by IBM Corporation, US.
Using the iSCSI makes the following possible: a disk that is located at the data center and connected to a client via a LAN or other communications lines seems to the client just like a local disk directly connected to the client's system. The advantages of the ASP are also true for the storage system using the iSCSI. The clients need not install large-capacity storage units in their system and can entrust data management such as backup to the data center, and thus can reduce the management cost. Migration of storage units such as disks included in the system that was run completely under the management of a client to the data center is possible without changing the existing application programs and the client can benefit from the reduced management cost. The manner in which businesses offer such storage management services is called a Storage Service Provider (SSP).
Meanwhile, one problem with the ASP and SSP is that the time from a data request until the data is received may be long due to communication delay as communication takes place between a client and the server for each transaction. Particularly in the sphere of medical information, large amounts of image data such as CT and MRI are often requested and communication delay becomes considerable. To hide communication delay, caching the data to be communicated is generally effective. However, using caches is effective for hiding communication delay only if cached data is reused, but ineffective when data is referenced just once.
As a method for hiding communication delay for data that is referenced just once, a prefetch cache method is known. For example, JP Serial No. 3074535 disclosed a method for hiding data access delay in which prefetch caches based on scheduling are used in a medical information system.
However, it is difficult to apply the method of hiding delay using prefetch caches to existing applications. This is because a prefetch program is provided separately from the existing applications and cached data is preserved in another application program than the existing applications. Accordingly, the existing applications determine whether data to access has been prefetched when accessing data and receive the data from the prefetch program if the data has been prefetched; if not, the applications must branch to request the OS to read or write the data.
In the method of hiding delay using prefetch caches, cached data is preserved in another application program than the existing applications. Therefore, it is difficult to apply this method to the existing applications.